Many Orthogonal Frequency Division Multiplexing (OFDM)-based systems, such as the Universal Mobile Telecommunications Standard Long Term Evolution (UMTS LTE), require that receivers of the system are able to process a received OFDM-signal to acquire a relatively high signal-to-noise ratio (SNR). The receiver should be able to process the signal without introducing impairments or noise (or at least without introducing impairments or noise that are of a severe nature). Further, the receiver should be able to adjust for impairments and/or noise introduced by the channel over which the received OFDM-signal was transmitted.
Furthermore, many OFDM-based systems employ complex transmission schemes, such as Multiple-Input Multiple-Output (MIMO) and/or large signal constellations, which may further increase the demands on the receiver.
To be able to meet such demands of high SNR in receivers operating in OFDM-systems, an expensive radio design may be required. Alternatively (or additionally) methods may be employed that are able to handle radio imperfections resulting from a non-optimal radio design.
One radio imperfection is IQ-imbalance. IQ-imbalance is one of the more limiting radio imperfections, and is thus important to dispose of or at least suppress.
IQ-imbalance may be generated by anything that affects the in-phase (I) and quadrature (Q) components of the received OFDM-signal differently. One example source of IQ-imbalance is a local oscillator of a receiver or a transmitter (or both). Another example source of IQ-imbalance is mismatch between one or more blocks in the respective I- and Q-paths of the receiver chain. Examples of blocks that may experience such mismatch are amplifiers and channel filters.
IQ-imbalance can be modeled, in the receiver, as a difference in phase and amplitude between the in-phase and quadrature oscillator components (i.e. the carriers). After down-converting the received signal to a baseband signal in down-conversion mixers, this difference in phase and amplitude results in a leakage between the in-phase and quadrature components of the baseband signal. Thus, the real part of the symbols will affect the imaginary part of the symbols, and vice versa;yIQ[n]=y[n]+ρy*[n],  (1)where * denotes conjugate, ρ represents the IQ-imbalance and is a factor that depends on the phase and amplitude mismatch (for example in the local oscillator or of blocks in the respective I- and Q-paths of the receiver chain), y[n] is what the received signal would have been if there was no IQ-imbalance, and yIQ[n] is the actually received signal.
In OFDM, data is transmitted in parallel on a number of sub-carriers (or sub-carrier frequencies), which may be efficiently implemented by using an inverse fast Fourier transform (IFFT) in the transmitter, and a fast Fourier transform (FFT) in the receiver. If the size of the FFT is N, then N samples at the output of the FFT are referred to as an OFDM-symbol (i.e. a frequency domain OFDM-symbol).
Each OFDM-symbol thus comprises data on N sub-carriers. Each such piece of data will be referred to as a symbol (in contrast to an OFDM-symbol), and may comprise a pilot symbol or an information symbol. In UMTS LTE, a symbol as described above may be denoted a resource element, and a pilot symbol may be denoted a reference signal.
In an OFDM-based system, the baseband signal is thus transformed, in the receiver, to a frequency domain signal and this is commonly achieved by applying an FFT to the baseband signal. When transformed to the frequency domain, the IQ-imbalance affects the frequency domain signal in frequency pairs. Thus, the symbols on sub-carrier N−k leak into sub-carrier k and vice versa. This may be expressed by the following frequency domain expression:YIQ(k)=Y(k)+ρY*(N−k),  (2)where Y(k) is what the received signal would have been if there was no IQ-imbalance, and YIQ(k) is the actually received signal. It may be noted that the notation of sub-carrier N−k is equivalent to sub-carrier −k. This is due to the N-periodicity of the FFT. Throughout this application, sub-carrier N−k will be denoted the mirror sub-carrier of sub-carrier k, and sub-carriers k and N−k will be denoted a frequency pair.
The leakage from a sub-carrier to another sub-carrier is a form of inter-carrier interference (ICI), and will degrade the SNR in the receiver. Thus, in order to achieve a high SNR while allowing for a less expensive radio design, it may be desirable to measure (or estimate) the IQ-imbalance and perform compensation on the received signal for the estimated IQ-imbalance. For example, the value ρ can be estimated. The estimated value {circumflex over (ρ)} can then be used to perform compensation on the received signal. The estimated value {circumflex over (ρ)} may, for example, be determined based on known pilot values and known channel values (e.g. channel estimates). If the value {circumflex over (ρ)} is accurately estimated, the compensation will cancel the leakage from the mirror sub-carrier completely.
Another imperfection experienced in wireless communication systems is frequency offset. A frequency offset experienced at a communication receiver may, for example, be due to mismatches between transmitter and receiver oscillators (which may in turn be caused by e.g. component mismatch, temperature variations, etc.) or Doppler phenomena.
Frequency offset compensation may be achieved in different ways. For example, the frequency of the local oscillator may be adjusted based on a detected or estimated frequency offset. Another possibility is to perform digital frequency offset compensation. Digital offset compensation may be achieved via a multiplication of the received baseband signal with a phase ramp, e.g. exp(j2π{circumflex over (υ)}n/N), where {circumflex over (υ)} is the normalized estimated frequency offset. A frequency offset compensator unit using this approach is commonly referred to as a digital rotator.
When digital frequency offset compensation is applied to the baseband signal, equations (1) and (2) will no longer be valid. Thus, if conventional IQ-imbalance estimation and compensation approaches were applied to such a signal, the result would not be optimal. The value {circumflex over (ρ)} would not be accurately estimated. Further, even if the value {circumflex over (ρ)} were accurately known, the compensation itself would not be optimal if conventional compensation techniques were used.
Similar problems may arise in wireless communication systems (and in receivers for such systems) not based on OFDM, but, for example, on SC-FDMA (Single Carrier Frequency Division Multiple Access).
Thus, there is a need for improved methods of and arrangements for IQ-imbalance compensation of a received wireless communication signal when frequency offset compensation is applied. Further, there is a need for improved methods of and arrangements for estimating IQ-imbalance in such cases.